1. Field of the Invention
The present invention relates to an optical film. An optical film of the present invention may be used independently or may be used in combination with other optical films as various optical films, such as retardation films, viewing angle compensation films, optical compensation films, elliptically polarizing plates, and brightness enhancement films. An optical film of the present invention is especially useful when it is laminated with polarizing plates to be used as elliptically polarizing plates.
In addition, the present invention relates to an image viewing display such as a liquid crystal display, an organic EL (electroluminescence) viewing display, a PDP using the optical film, and the elliptically polarizing plate. Especially, the optical film, and polarizing optical film of the present invention may be suitably used for liquid crystal displays that can be mounted in portable information and telecommunications instruments, personal computers, and the like. Above all, they are suitable for liquid crystal displays operating in so-called IPS mode.
2. Description of the Related Art
Conventionally, many optical films comprising various kinds of polymer materials have been used for the purpose of improving picture-quality in image viewing displays, such as portable information and telecommunications instruments, liquid crystal monitors, liquid crystal televisions, organic EL viewing displays. For example, performing stretching process for polymer films having birefringence produces such optical films. Among them when a direction wherein refractive indexes of refraction in a film plane gives a maximum is defined as an X-axis, a direction perpendicular to the X-axis is defined as a Y-axis, a thickness direction of a film is defined as a Z-axis, and refractive indexes in each axial direction are defined as nx, ny, nz, respectively, an optical film wherein a Nz coefficient expressed in a formula of (nx−nz)/(nx−ny) is controlled may preferably be used in order to widen a viewing angle of image viewing displays, such as the above-mentioned liquid crystal displays.
An Nz coefficient preferable for optical films depends upon modes (TN, VA, OCB, IPS modes, etc.) of the liquid crystal displays. Therefore, in order to obtain optical films having a required Nz coefficient polymer materials having superior film workability and birefringence that may easily be controlled into a desired Nz coefficient are suitably selected for use. For example, since optical films satisfying a relationship of Nz coefficient≦0.9 may control indexes of refraction to be at least nz>ny, polymer materials having such indexes of refraction and developing birefringence are suitably used.
Since optical films satisfying Nz coefficient≦0.9 may advantageously developing superior birefringence, for example, they may be obtained by stretching polycarbonate resin films including a unit of 2,2-bis (4-hydroxyphenyl) propane as polymer films (See Japanese Patent Laid-Open No. 5-157911 official report). The polycarbonate resins are preferable from a viewpoint of having high transparency and moderate heat resistance. However, optical films obtained by stretching of polycarbonate resin films have a large birefringence change when stress is applied; that is, they have a large photoelastic coefficient. Therefore, there occurs a problem that the optical films easily cause a large degree of unevenness when they are adhered to polarizing plates. Moreover, in recent years, upsizing of liquid crystal panels, such as in liquid crystal televisions, increases stress that works on panels, and therefore optical film materials having smaller change of retardation (change of birefringence) is increasingly required. Moreover, the optical films have such problems that exhibit large retardation variation, under use environment after adhered onto viewing displays. Since they had this problem, the optical films were not suitable for application in recent years wherein high heat resistance, and high temperature and high moisture resistance were required.
On the other hand, as polymer materials having a comparatively small photoelastic coefficient, for example, norbornene resins are known (See Japanese Patent Laid-Open No. 2000-56131 official report). However, although the norbornene resins have a small photoelastic coefficient, they simultaneously show a characteristic to have a small birefringence, providing a limitation to retardation given by stretching process. Especially, control of three dimensional refractive index satisfying a relationship of Nz coefficient≦0.9 is difficult.
And conventionally, as a liquid crystal display, there has been mainly used a liquid crystal display in so-called TN mode in which a liquid crystal having a positive dielectric anisotropy is twisted and horizontally oriented between substrates mutually facing to each other. However, in TN mode, even if black display is required, optical leakage resulting from birefringence caused by liquid crystal molecules near a substrate made it difficult to realize perfect display of black color owing to driving characteristics thereof. On the other hand, in a liquid crystal display in IPS mode, since liquid crystal molecules have almost parallel and homogeneous orientation with respect to a substrate surface in non-driven state, light passes through the liquid crystal layer, without giving almost any change to a polarization plane, and as a result, arrangement of polarizing plates on upper and lower sides of the substrate enables almost perfect black display in non-driven state.
Although almost perfect black display may be realized in normal direction to a panel in IPS mode, when a panel is observed in a direction shifted from normal direction, inevitable optical leakage occurs caused by characteristics of a polarizing plate in a direction shifted from an optical axis of the polarizing plates disposed on upper and lower sides of the liquid crystal cell, as a result, causing to a problem of narrowing of a viewing angle.
In order to solve this problem, a polarizing plate is used in which the shift of a geometric axis of a polarizing plate given when observed from oblique direction is compensated by a retardation film. Polarizing plates providing such effect are disclosed (See Japanese Patent Laid-Open No. 4-305602, official report). However, retardation films conventionally known have not been able to easily realize sufficient wide viewing angles.